A well-known data network is Internet. In an IP domain (Internet Protocol), the data network is open, which means that any terminals may communicate two-by-two, for example by using a same signaling protocol. Multimedia type sessions may thereby be created via the Internet network between two communication terminals. Convergence between voice networks and data networks is allowed by using suitable signaling protocols.
Voice technology on IP or VoIP and more generally technologies allowing establishment of multimedia sessions most frequently use the SIP protocol (Session Initiation Protocol), which is an interoperable open standard. Other signaling protocols, for example H323, MGCP (Media Gateway Control Protocol) and Megaco (this last protocol having been selected in the UMTS standard by 3GPP for controlling Media Gateways) may also be used for multimedia sessions.
The SIP protocol is standardized by the IETF (Internet Engineering Task Force) and in particular described by RFC 3261. The SIP protocol, like the H323 and MGCP protocols, was designed for establishing, modifying and terminating multimedia sessions. It is responsible for authentication and localization of multiple participants. It is also responsible for negotiation on the types of media which may be used by the different participants, by encapsulating SDP (Session Description Protocol) messages. The SIP protocol does not transport the data exchanged during the session such as voice or video. As this protocol is independent of the transmission of the data, any type of data and protocols may be used for this exchange: this is the RTP protocol (Real-time Transport Protocol) which most frequently provides audio and video sessions. A benefit from the SIP protocol is that it is not only intended for voice on IP but also for many other applications such as video telephony, instantaneous messaging, virtual reality or even video games.
As any signaling protocol, SIP incorporates a declaration phase at the home network, and phases for requesting establishment of a multimedia session, of negotiation of the features of the requested service and finally a phase for closing the multimedia session. The session initiation protocol (SIP) from version v2.0 onwards allows information exchange between the communicating entities before or during the multimedia session. In a way known per se, these exchanges may be achieved through the following methods:
INFO: allows exchange of information not affecting the state of the call (as described in RFC 2976). In certain cases, this field is used for transferring DTMF tones.
NOTIFY: allows the sending of event notifications (RFC 3265).
SUBSCRIBE: allows subscription to event notifications (as described in RFC 3265).
UPDATE: is used for updating the media parameters (cf. RFC 3311);
MESSAGE: this method defined in RFC 3428 <<SIP extension for Instant Messaging>> allows exchange of instantaneous messages between two terminals.
In a radiotelephone network for example, by using such protocols (SIP/H323/MGCP) for multimedia sessions, it is possible to exchange information through data channels parallel to voice channels. But the drawback of these information exchange methods is that they are not always supported by the underlying networks and/or simple terminals.